The present invention relates to a shift control system and method for a V-belt type continuously variable transmission and particularly to a control of a shift actuator at start of an engine, which shift actuator is used in the shift control system for actuating a shift control valve.
Generally, in such a continuously variable transmission, an oil pressure (hereinafter referred to as a primary pulley pressure) that is obtained by regulating a line pressure serving as a base pressure is regulated by means of a shift control valve, is supplied to a primary pulley chamber. The primary pulley pressure is increased or decreased by means of the shift control valve for thereby varying a groove width of the primary pulley. By varying the groove width of the primary pulley, a diameter ratio between the primary pulley and a secondary pulley is varied, thus enabling the transmission ratio of the transmission to vary continuously. The shift control valve is connected to a middle portion of a shift link that constitutes a mechanical feedback mechanism. To the opposite ends of the shift link are connected a movable sheave of the primary pulley and a step motor serving as a shift actuator, respectively. In order to prevent a discrepancy between a rotational position of the step motor that causes the shift control valve to stroke into a position corresponding to a command value for attaining a desired transmission ratio and the command value supplied to the step motor, the step motor is initialized during stop of a vehicle or at start of the engine.
Initialization performed by a conventional system is for causing the command value supplied to the step motor that operates the shift control valve to exactly correspond to the rotational position of the step motor. An example of such initialization is that when an ignition switch is turned on a motor initializing operation means causes the step motor to move in one direction into a hardware limit position and thereafter return to a standard position through movement in the opposite direction. When an initializing operation of the step motor is finished, a motor command value initializing means initializes the motor command value so that the motor command value corresponds to the standard position. By this, the motor command value can exactly corresponds to the rotational position of the step motor and therefore an accurate shift control can be attained (refer to Unexamined Japanese Patent Publication No. 8-178063).